Autism spectrum disorders are defined by aberrant development of communication and socialization in the presence of restrictive and/or repetitive behaviors. Recent epidemiologic studies have documented an increase in the number of children identified with autism spectrum disorder over the past decade and according to some, the current numbers indicate a prevalence of 1 per 150 (CDC, MMWR 2007, Feb 9th release) Despite the pressing need to identify causal factors, etiology remains elusive. Furthermore, the heterogeneity of presentation complicates attempts to locate autisms home in the brain. [unreadable] Polysomnography (PSG) is a reliable non-invasive tool that can be used to study the basic pathophysiological mechanisms of autism and other developmental and neuropsychiatric disabilities. Our preliminary data in young children with autism support a growing body of literature demonstrating that sleep architecture is abnormal in this disorder. Previous studies in children with autism have identified various abnormalities in REM sleep including the following: immature organization, decreased quantity, abnormal twitches, undifferentiated sleep and REM sleep behavior disorder characterized by the absence of the muscle atonia that is normal in REM sleep and resulting in an acting out of dreams phenomenon. Preliminary observations from another PDN project (MH002868-03) revealed that children with autism spent an abnormally short time in the REM sleep stage of sleep compared to total sleep time (hereafter referred to as SPT REM% for REM sleep as a percent of sleep period time), and had a prolonged latency to REM sleep. [unreadable] The function of REM sleep and its relationship to cognition and overall neurological health is unknown and a subject of ongoing research. We know from animal studies that REM sleep increases after intensive learning sessions. These laboratory findings formed the basis for the hypothesis that this sleep stage is important for cognitive processes and that REM sleep may be useful as an indicator of brain plasticity. REM sleep has most recently been implicated in the process of human memory consolidation and several studies suggest that it is crucial to normal cognitive function and in the processing of emotion in memory systems. [unreadable] Acetylcholine (Ach) is one of the major neurotransmitters necessary for normal sleep transitions and abnormalities in Ach have been implicated in REM deficient sleep in other populations, most notably Alzheimers disease. Donepezil hydrochloride has been shown to normalize REM sleep in these populations and we postulated that it would have similar beneficial effects in children with autism.[unreadable] We are conducting a single arm, open-label clinical trial of donepezil hydrochloride to evaluate its ability to enhance REM sleep in children with autism who exhibit a low SPT REM% (defined as below 2 standard deviations of observed normative data for age). All patients will come through the screening protocol 06-M-0065. Those who meet a research diagnosis of autism spectrum disorder and are ages 2 to 11 (through the tenth year) will be evaluated for inclusion/exclusion criteria for the study. The trial is open and we are actively recruiting participants.[unreadable] The primary outcome measure of this protocol is to increase the SPT REM% in children with autism such that their REM/non-REM ratios begin to approach normative values. Donepezil enhanced REM sleep has been achieved in young healthy adults, in elderly, healthy adults and in elderly, demented adults with Alzeheimers disease. Furthermore, the studies in Alzheimer disease by Mizuno et al showed a positive correlation between improved cognition and increased SPT REM %. If REM sleep is necessary for normal cognition, and its deficiency or absence can be remedied by pharmacologic intervention, then it may follow that improvement of REM sleep correlates with improved short and long term cognition in children with autism.